The invention relates to lithography, and in particular, to optical and process correction techniques for lithography tools.
The fabrication of integrated circuits on a semiconductor substrate typically includes multiple photolithography steps. A photolithography process typically begins by applying a thin layer of a photoresist material to the substrate surface. The photoresist is then exposed through a photolithography tool to a radiation source that changes the solubility of the photoresist at areas exposed to the radiation. The photolithography tool typically includes transparent regions that do not interact with the exposing radiation and a patterned material or materials that interact with the exposing radiation, either to block it or to shift its phase.
Areas of the photoresist that are not exposed to the radiation do not change in solubility, so those unexposed areas (if xe2x80x9cnegative photoresistxe2x80x9d is used), or the exposed areas (if xe2x80x9cpositive photoresistxe2x80x9d is used) can be washed away by a developer, leaving patterned photoresist on the substrate. The patterned photoresist is then used as a protective layer during a subsequent fabrication step, such as etching an underlying layer or diffusing atoms into unmasked areas of the substrate.
xe2x80x9cMasksxe2x80x9d and xe2x80x9creticlesxe2x80x9d are types of lithography exposure tools, that is, tools that alter radiation to print an image. The term xe2x80x9cmaskxe2x80x9d is sometimes reserved for photolithography exposure tools that print an entire wafer in one exposure, and the term xe2x80x9creticlexe2x80x9d is sometimes reserved for a photolithography exposure tool that projects a demagnified image and prints less than the entire wafer during each exposure. The term xe2x80x9cmaskxe2x80x9d is more commonly used generically, however, to refer to any type of lithography exposure tool. The term xe2x80x9cmaskxe2x80x9d is used herein in its broadest sense to mean any type of lithography exposure tool, regard less of the magnification, the type of exposing radiation, the fraction of the wafer that is printed in each exposure, or the method, such as reflection, refraction, or absorption, used to alter the incoming radiation.
A photolithography mask typically comprises a quartz substrate with a patterned layer of opaque chromium that corresponds to the circuit pattern to be transferred to the substrate. A mask can also include a material, such as silicon nitride, that shifts the phase of the exposing radiation. A reduced image of the mask is typically projected onto the substrate, the image being stepped across the substrate in overlapping steps to repeat the pattern.
As each successive generation of integrated circuits crowd more circuit elements onto the semiconductor substrate, it is necessary to reduce the size of the features, that is, the lines and spaces that make up the circuit elements. The minimum feature size that can be accurately produced on a substrate is limited by the ability of the fabrication process to form an undistorted optical image of the mask pattern onto the substrate, the chemical and physical interaction of the photoresist with the developer, and the uniformity of the subsequent process, e.g., etching or diffusion, that uses the patterned photoresist.
When a photolithography system attempts to print circuit elements having sizes near the wavelength of the exposing radiation, the printed circuit elements becomes significantly different from the pattern on the mask. For example, line-widths vary depending on the proximity of other lines. The inconsistent line widths can then cause circuit components that should be identical to operate at different speeds, thereby creating problems with the overall operation of the integrated circuit. As another example, lines tend to shorten, that is, the line ends xe2x80x9cpull back.xe2x80x9d The small amount of shortening becomes more significant as the lines themselves are made smaller. Pulling back of the line ends can cause connections to be missed or to be weakened and prone to failure.
Because of the wave nature of light, even a perfectly straight, opaque edge will not produce a shadow that is absolutely dark in the shadowed areas. A phenomenon known as diffraction causes the light to bend around an edge to produce a pattern of alternating light and dark areas. The extent of the alternating areas is on the order of the wavelength of the exposing light and the diffraction pattern intensity falls off rapidly in the shadowed zone. When integrated circuits used conductor widths greater than one micron, the effect of diffraction was small and the differences between the pattern on the mask and the pattern produced on the substrate could be ignored. In modern circuits, with conductors widths well under a micron and even under two tenths of a micron, diffraction and other optical phenomena produce effects that are significant in relation to the size of features being produced by photolithography, and such effects can no longer be ignored.
Because the size of the diffraction effects is related to the wavelength of light used, one way to reduce diffraction effects is to use light having a smaller wavelength. The wavelengths used in new photolithography systems have decreased over the years from visible light to ultra violet to deep ultraviolet. Systems using extreme ultra-violet or soft x-rays are currently being developed. It is desirable, however, to improve the resolution of existing photolithography systems because of the high cost of new systems and because it takes many years for a new generation of photolithography systems to become stable production tools. Moreover, the rate at which shorter wavelength systems are being developed is expected to be insufficient to keep up with the expected reduction in circuit feature size. Thus, it will likely be necessary to overcome diffraction effects, regardless of the wavelength used.
Because it can be determined in many instances how the pattern projected onto the substrate will vary from the mask pattern, the mask pattern can be altered to pre-compensate for the distortion. The projected pattern, rather than the mask pattern itself, then portrays the desired circuit. Techniques for pre-compensating the mask are examples of resolution-enhancing corrections or resolution enhancement techniques. The mask is typically altered by moving features of the mask or adding xe2x80x9csub-resolutionxe2x80x9d assist features, that is, features that are too small to be imaged individually on the substrate, but that scatter or bend light to alter the image of other, larger features on the mask. These xe2x80x9cpredistortionsxe2x80x9d cancel the distortions inherent in the lithography process, resulting in a layout that has improved fidelity to the intended design, improved manufacturing yield, and better circuit performance.
For example, it is known that diffraction effects tend to round off square corners and shorten lines. FIG. 1A shows a pattern 100 on a portion of a mask 102, and FIG. 1B shows the pattern 104 printed by mask 102 onto a substrate 106. Printed pattern 104 is shorter than mask pattern 100 and printed pattern 104 has rounded corners. FIG. 1C shows a modified mask pattern 108 having xe2x80x9cserifsxe2x80x9d 110 added. FIG. 1D shows the pattern 116 projected onto a substrate 118 by mask pattern 108 using serifs 110. Printed pattern 116 is not as shortened as pattern 104 in FIG. 1B and the corners are not as rounded. The use of serifs was described as early as 1981 by B. E. A. Saleh and S. Sayegh in xe2x80x9cReduction of Error of Microphotographic Reproductions by Optimal Correction of Original Masks,xe2x80x9d Opt. Eng., vol. 20, p. 781, and is described more recently, for example, in U.S. Pat. No. 5,707,765 to Chen for xe2x80x9cPhotolithography Mask Using Serifs and Method Thereof.xe2x80x9d
It is also known that the diffraction patterns of closely spaced mask pattern features interact. For example, FIG. 2A shows a group of closely spaced parallel lines 202 and an isolated line 206. Isolated line 206 will print a line having a width different from that of lines printed by closely spaced lines 202. Non-uniform thickness in printed lines can interfere with circuit device functioning as described above. Isolated lines can be made to print like the closely spaced lines by adding xe2x80x9cscattering barsxe2x80x9d 210 (FIG. 2B), that is, additional lines on the mask on opposite sides of the isolated line. Scattering bars are described, for example, in U.S. Pat. No. 5,821,014 to Chen et al. for xe2x80x9cOptical Proximity Correction Method for Intermediate Pitch Features Using Sub-Resolution Scattering Bars on a Mask.xe2x80x9d Note that a scattering bar 210 is also used along the outside edge of the line 202A, the last line in the closely spaced group. Whether or not a scattering bar is necessary along a particular edge of a feature depends upon the distance to the closest facing feature edge.
Scattering bars, like the serifs described above, are sufficiently thin that they are below the resolution limit of the lithography system and therefore do not appear or xe2x80x9cprintxe2x80x9d on the photoresist. These features do, however, affect the printed image of the nearby features and can make the printed image of the isolated or outside lines, such as line 202A, consistent with the images of the closely spaced lines. Single or multiple scattering bars can be used on both sides of an isolated line.
FIG. 2C shows a portion of a mask 218 having another type of assist feature, anti-scattering bars 220. Anti-scattering bars 220 are actually transparent lines created in an otherwise opaque region 222 of the mask. Many types of assist features have been developed. The following is an exemplary, but not exhaustive, list of design structures that can benefit from the addition of assist features: line ends, line corners, isolated lines, isolated lines adjacent to a set of dense lines, cross or xe2x80x9cXxe2x80x9d structures, and transistor gates.
During the design of masks for fabricating integrated circuits, resolution enhancement techniques are often implemented automatically by automated design tool. The automated design tools review the mask design to locate mask elements that would benefit from a resolution enhancement technique and automatically applies the appropriate technique to the mask. There are two basic strategies used to determine the need to apply a resolution enhancement technique. The first strategy is rule based, and compares the mask features with a set of rules. The second strategy is a model-based.
In a rule-based system, the design is checked against a list of rules, and a resolution enhancement technique is applied when a rule indicates that one is required. Rule-based systems are simpler and use considerably less computing resources than do model-based systems. Because of the large number of possible combination of mask feature patterns and the relatively small number of rules, the rules do not correct the mask equally well in all situations, and techniques may be added where they are not necessary, thereby unnecessarily increasing the mask complexity.
In a model-based strategy, the image that would be produced by a mask pattern is determined using a software model of the mask, and resolution enhancement techniques are applied only where the model shows them to be needed. Model based strategies are more effective, but require more computational resources.
On a typical integrated circuit, groups of features are repeated multiple times throughout the design. Designers simplify the design process by treating groups of features as standard cells and reusing them throughout the design. A design can, therefore, be analyzed at different levels, that is, as a hierarchy of building blocks or xe2x80x9cflattenedxe2x80x9d as a collection of individual polygons. When a mask is being fabricated, the design is flattened to a collection of polygons. Resolution enhancement techniques can be applied at any level of the design hierarchy. It is typically more efficient to apply the techniques at a higher, hierarchical level, so that they need be applied only once for each type of cell. When resolution enhancement techniques are applied to standard cell, however, it may stilt be necessary to consider how features at the edges of the cells interact with nearby features outside the cell.
When automatic design tools detect the need for resolution enhancement techniques and apply them to a mask, there are sometimes conflicts between the additions or alterations required by features of the mask. A conflict exists when the combination of mask additions or alterations leads to undesirable results on the photoresist or on the mask itself. For example, if overlapping scattering bars were added, they could cause unwanted features to be printed on the substrate. Alternatively, the placement of some assist features could produce a mask having features that are difficult to manufacture or inspect.
FIG. 3A shows a portion of mask 300 having a feature 302 that can benefit from the addition of an assist feature, and shows scattering bars 306 and 308 as determined by a design program. The intersection of scattering bars 306 and 308 could cause an artifact to print on the substrate and the small length of scattering bars 306 and 308 that extend past the other scattering bar may be interpreted to be a defect by automated mask inspection software. Scattering bar 306 and 308 are said to interact, and because the interaction is unintended and harmful, they are said to interfere.
FIG. 3B shows one way a program could handle the conflict. The program first analyzes edge 312, determines that a scattering bar is necessary, and adds scattering bar 306. Next the program continues with its analysis and determines that a scattering bar is necessary along edge 314. Because a scattering bar along edge 314 would intersect with scattering bar 306, however, the program does not add a scattering bar along edge 314. Thus, the conflict between scattering bars is resolved not on the basis of which assist feature is more important for producing an accurate printed image, but upon which edge the design program happened to analyze first. FIG. 3C shows another method of handling conflicts, in which the second scatter bar 308, rather than being removed entirely, is shortened to remove the overlap. FIG. 3D shows yet another method of handling conflicting scattering bars of FIG. 3A, in which both scattering bars 306 and 308 are truncated to remove the interference. As in the technique described with respect to FIG. 3B, the second scatter bar analyzed is altered, regardless of its relative importance. This technique also can result in the reduction of a critical assist feature to perpetuate a less critical one.
An object of the invention is to improve lithography tools to improve the fidelity of the lithography process.
The present invention comprises a method and apparatus for implementing resolution-enhancing corrections in lithography tools. In accordance with the invention, resolution-enhancing corrections are assigned priorities. Conflicting resolution-enhancing corrections are identified, and the conflicting resolution-enhancing correction having the lower priority is altered to eliminate the conflict. Prioritizing conflicting resolution-enhancing corrections produces a lithography tool having improved fidelity because resolution-enhancing corrections that provide the most beneficial effects can be implemented at the expense of resolution-enhancing corrections that provide less benefit. In a preferred embodiment, the prioritization is based on the geometry of the conflicting resolution-enhancing corrections or of the pattern features that generate the corrections, rather than on the function of the circuit element corresponding to the mask feature.
The invention includes not only methods for designing a mask, but also software for implementing the methods, a computer programmed to carry out the methods, a computer implementable description of a mask design determined by application of the methods, the fabrication of a mask designed by the methods, and the mask designed by the methods.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.